Semiconductor device and method of controlling same

ABSTRACT

A semiconductor device includes a semiconductor part having a first surface and a second surface opposite to the first surface, a first electrode on the first surface, a second electrode on the second surface, first to third control electrodes between the first electrode and the semiconductor part. The first to third control electrodes are biased independently from each other. The semiconductor part includes a first layer of a first-conductivity-type, a second layer of a second-conductivity-type, a third layer of the first-conductivity-type and the fourth layer of the second-conductivity-type. The second layer is provided between the first layer and the first electrode. The third layer is selectively provided between the second layer and the first electrode. The fourth layer is provided between the first layer and the second electrode. The second layer opposes the first to third control electrode with insulating films interposed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/383,837 filed on Jul. 23, 2021 which is a divisional of U.S.application Ser. No. 16/573,593 filed on Sep. 17, 2019 (now U.S. Pat.No. 11,101,375) and is based upon and claims the benefit of priorityfrom Japanese Patent Application No. 2019-050702, filed on Mar. 19,2019, and Japanese Patent Application No. 2019-166842, filed on Sep. 13,2019; the entire contents of all of which are incorporated herein byreference.

FIELD

Embodiments relate to a semiconductor device and a method of controllingthe same.

BACKGROUND

For example, an insulated gate bipolar transistor (hereinbelow, IGBT) isused as a semiconductor device controlling a large current under thehigh voltage of 600 V or more. For example, IGBT is used for powerconversion in which both a low steady loss (a low on-resistance) and alow switching loss (a fast switching speed) are desirable to increasethe conversion efficiency.

To reduce the on-resistance, many IGBTs of recent years have a trenchgate structure extending deeply into the n-type base layer through thep-type base layer. Thereby, the channel density can be increased;carriers inside the n-type base layer can be accumulated efficiently byutilizing the configurations of the trench gates that aremutually-adjacent inside the n-type base layer; and the on-resistancecan be reduced in the steady state. However, when a low on-resistance isrealized by increasing the amount of the accumulated carriers, thecarrier amount to be ejected is high while turning off. Therefore, theturn-off time lengthens; and the turn-off loss increases. In otherwords, there is a trade-off relationship between the decrease of theon-resistance and the decrease of the turn-off loss.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a semiconductordevice according to an embodiment;

FIG. 2 is a time chart showing an operation of the semiconductor deviceaccording to the embodiment;

FIGS. 3A to 3C are schematic views showing the operation of thesemiconductor device according to the embodiment;

FIG. 4 is a schematic cross-sectional view showing a semiconductordevice according to a modification of the embodiment;

FIG. 5 is a schematic cross-sectional view showing a semiconductordevice according to another modification of the embodiment;

FIG. 6 is a schematic cross-sectional view showing a semiconductordevice according to yet another modification of the embodiment;

FIG. 7 is a schematic cross-sectional view showing a semiconductordevice according to other modification of the embodiment;

FIG. 8 is a time chart showing an operation of the semiconductor deviceaccording to the other modification of the embodiment; and

FIGS. 9A to 9C are schematic views showing the operation of thesemiconductor device according to the other modification of theembodiment

DETAILED DESCRIPTION

According to one embodiment, a semiconductor device includes asemiconductor part including a first semiconductor layer of a firstconductivity type, the semiconductor part having a first surface and asecond surface, the second surface being positioned on a side oppositeto the first surface; a first electrode provided on the first surface; asecond electrode provided on the second surface; a first controlelectrode provided between the first electrode and the semiconductorpart; a second control electrode provided between the first electrodeand the semiconductor part; and a third control electrode providedbetween the first electrode and the semiconductor part. The firstcontrol electrode is electrically insulated from the semiconductor partby a first insulating film, and electrically insulated from the firstelectrode by a second insulating film. The second control electrode iselectrically insulated from the semiconductor part by a third insulatingfilm, and electrically insulated from the first electrode by a fourthinsulating film. The second control electrode is biased independentlyfrom the first control electrode. The third control electrode iselectrically insulated from the semiconductor part by a fifth insulatingfilm, and electrically insulated from the first electrode by a sixthinsulating film. The third control electrode is biased independentlyfrom the first control electrode and the second control electrode. Thesemiconductor part further includes a second semiconductor layer of asecond conductivity type, a third semiconductor layer of the firstconductivity type, and a fourth semiconductor layer of the secondconductivity type. The second semiconductor layer is provided betweenthe first semiconductor layer and the first electrode. The thirdsemiconductor layer is selectively provided between the secondsemiconductor layer and the first electrode. The fourth semiconductorlayer is provided between the first semiconductor layer and the secondelectrode. The second semiconductor layer opposes the first controlelectrode with the first insulating film interposed. The secondsemiconductor layer opposes the second control electrode with the secondinsulating film interposed. The second semiconductor layer opposes thethird control electrode with the third insulating film interposed.

Embodiments will now be described with reference to the drawings. Thesame portions inside the drawings are marked with the same numerals; adetailed description is omitted as appropriate; and the differentportions are described. The drawings are schematic or conceptual; andthe relationships between the thicknesses and widths of portions, theproportions of sizes between portions, etc., are not necessarily thesame as the actual values thereof. The dimensions and/or the proportionsmay be illustrated differently between the drawings, even in the casewhere the same portion is illustrated.

There are cases where the dispositions of the components are describedusing the directions of XYZ axes shown in the drawings. The X-axis, theY-axis, and the Z-axis are orthogonal to each other. Hereinbelow, thedirections of the X-axis, the Y-axis, and the Z-axis are described as anX-direction, a Y-direction, and a Z-direction. Also, there are caseswhere the Z-direction is described as upward and the direction oppositeto the Z-direction is described as downward.

FIG. 1 is a schematic cross-sectional view showing a semiconductordevice 1 according to an embodiment. The semiconductor device 1 is, forexample, an IGBT. The impurity distribution and the value of theimpurity concentration of each semiconductor layer in the specificationcan be measured using, for example, secondary ion mass spectrometry(SIMS) or spreading resistance analysis (SRA). The relative high/lowrelationship of the impurity concentrations of two semiconductor layerscan be determined using, for example, scanning capacitance microscopy(SCM). The relative high/low relationship and/or the absolute value ofthe carrier concentration of the semiconductor region are determined bySCM and SRA. By assuming the activation rate of the impurity, it ispossible to determine the absolute value of the impurity concentration,the distribution of the impurity concentration, and the relativehigh/low relationship between the impurity concentrations of twosemiconductor layers from the measurement results of SCM and SRA.

As shown in FIG. 1 , the semiconductor device 1 includes a semiconductorpart 10, an emitter electrode 20 (a first electrode), a collectorelectrode 30 (a second electrode), a first gate electrode 40, a secondgate electrode 50, and a third gate electrode 60.

The semiconductor part 10 is, for example, silicon. The semiconductorpart 10 has a first surface 10T and a second surface 10B. The secondsurface 10B is the back surface of the first surface 10T. The emitterelectrode 20 is provided on the first surface 10T. The collectorelectrode 30 is provided in contact with the second surface 10B. Theemitter electrode 20 and the collector electrode 30 include, forexample, at least one selected from the group consisting of aluminum(Al), titanium (Ti), nickel (Ni), tungsten (W), gold (Au), andpolysilicon.

The first gate electrode 40 is provided between the semiconductor part10 and the emitter electrode 20. For example, the first gate electrode40 is disposed inside a trench GT1 provided in the first surface 10Tside of the semiconductor part 10. The first gate electrode 40 extendsin the Y-direction inside the trench GT1. The first gate electrode 40 iselectrically insulated from the semiconductor part 10 by a gateinsulating film 41. Also, the first gate electrode 40 is electricallyinsulated from the emitter electrode 20 by an insulating film 43. Thefirst gate electrode 40 includes, for example, conductive polysilicon.The gate insulating film 41 and the insulating film 43 are, for example,silicon oxide films.

The second gate electrode 50 is provided between the semiconductor part10 and the emitter electrode 20. For example, the second gate electrode50 is disposed inside a trench GT2 provided on the first surface 10Tside of the semiconductor part 10. The second gate electrode 50 extendsin the Y-direction inside the trench GT2. The second gate electrode 50is electrically insulated from the semiconductor part 10 by a gateinsulating film 51. Also, the second gate electrode 50 is electricallyinsulated from the emitter electrode 20 by an insulating film 53. Thesecond gate electrode 50 includes, for example, conductive polysilicon.The gate insulating film 51 and the insulating film 53 are, for example,silicon oxide films.

The third gate electrode 60 is provided between the semiconductor part10 and the emitter electrode 20. For example, the third gate electrode60 is disposed inside a trench GT3 provided on the first surface 10Tside of the semiconductor part 10. The third gate electrode 60 extendsin the Y-direction inside the trench GT3. The third gate electrode 60 iselectrically insulated from the semiconductor part 10 by a gateinsulating film 61. Also, the third gate electrode 60 is electricallyinsulated from the emitter electrode 20 by an insulating film 63. Thethird gate electrode 60 includes, for example, conductive polysilicon.The gate insulating film 61 and the insulating film 63 are, for example,silicon oxide films.

The first gate electrode 40 is electrically connected to a first gatepad 47 via a first gate interconnect 45. The second gate electrode 50 iselectrically connected to a second gate pad 57 via a second gateinterconnect 55. The third gate electrode 60 is electrically connectedto a third gate pad 67 via a third gate interconnect 65. In other words,the first gate electrode 40, the second gate electrode 50, and the thirdgate electrode 60 are biased independently.

The first gate electrode 40 and the second gate electrode 50 each arearranged periodically in a direction along the first surface 10T of thesemiconductor part 10 (e.g., the X-direction). In the example shown inFIG. 1 , the first gate electrode 40 and the second gate electrode 50are arranged adjacent to each other in the X-direction. For example, atleast one of the third gate electrodes 60 is disposed between the firstgate electrode 40 and the second gate electrode 50. In the example, twoof the third gate electrodes 60 are disposed between the first gateelectrode 40 and the second gate electrode 50. Three or more of thethird gate electrodes 60 may be disposed between the first gateelectrode 40 and the second gate electrode 50.

The semiconductor part 10 includes an n-type base layer 11 (a firstsemiconductor layer), a p-type base layer 13 (a second semiconductorlayer), an n-type emitter layer 15 (a third semiconductor layer), ap-type contact layer 17, an n-type buffer layer 19 (a fifthsemiconductor layer), and a p-type collector layer 21 (a fourthsemiconductor layer).

The n-type base layer 11 includes, for example, n-type impurities with aconcentration range of 1×10¹² to 1×10¹⁵ (atoms/cm³). The n-type baselayer 11 has an n-type carrier concentration at which a prescribedbreakdown voltage can be achieved. Here, the n-type carrierconcentration is, for example, the value obtained by subtracting thep-type impurity concentration from the n-type impurity concentration.For example, the p-type impurities with a background-level are includedin the n-type silicon wafer used to form the n-type base layer 11. Forexample, the n-type base layer 11 has a thickness in the Z-direction inthe range of 1 to 1000 μm and is set to a thickness at which theprescribed breakdown voltage can be achieved.

The p-type base layer 13 is selectively provided between the n-type baselayer 11 and the emitter electrode 20. The p-type base layer 13 iselectrically connected to the emitter electrode 20. For example, thep-type base layer 13 is provided to have a p-type impurity amount in therange of 1×10¹² to 1×10¹⁴ cm⁻² and has a thickness in the Z-direction of0.1 to several μm. For example, the p-type base layer 13 is formed byion-implanting the p-type impurities on the first surface 10T side ofthe semiconductor part 10.

The n-type emitter layer 15 is selectively provided between the p-typebase layer 13 and the emitter electrode 20. The n-type emitter layer 15includes n-type impurities with a higher concentration than theconcentration of n-type impurities in the n-type base layer 11. Forexample, the emitter electrode 20 contacts the n-type emitter layer 15and is electrically connected to the n-type emitter layer 15.

For example, the n-type emitter layer 15 is provided to have an n-typeimpurity amount in the range of 1×10¹⁴ to 1×10¹⁶ cm⁻² and has athickness in the Z-direction of 0.1 to several μm. For example, then-type emitter layer 15 is formed by selectively ion-implanting then-type impurity on the first surface 10T side of the semiconductor part10.

The p-type contact layer 17 is selectively provided between the p-typebase layer 13 and the emitter electrode 20. The p-type contact layer 17includes p-type impurities with a higher concentration than theconcentration of p-type impurities in the p-type base layer 13. Forexample, the emitter electrode 20 contacts the p-type contact layer 17and is electrically connected to the p-type contact layer 17. The p-typebase layer 13 is electrically connected to the emitter electrode 20 viathe p-type contact layer 17.

For example, the p-type contact layer 17 is provided to have a p-typeimpurity amount in the range of 1×10¹⁴ to 1×10¹⁶ cm⁻² and has athickness in the Z-direction in the range of 0.1 to several μm. Forexample, the p-type contact layer 17 is formed by selectivelyion-implanting the p-type impurities on the first surface 10T side ofthe semiconductor part 10.

For example, the n-type emitter layer 15 and the p-type contact layer 17may be arranged alternately in the longitudinal direction of thetrenches GT1 to GT3 (e.g., the Y-direction). The ratio of the surfaceareas of the p-type contact layer 17 and the n-type emitter layer 15,which are exposed at the first surface 10T of the semiconductor part 10,can be changed freely according to the desired design. Moreover, then-type emitter layer 15 and the p-type contact layer 17 may have thesurfaces that have freely designed configuration exposed at the firstsurface 10T of the semiconductor part 10.

The trenches GT1 to GT3 have depths capable of reaching the n-type baselayer 11 from the first surface 10T of the semiconductor part 10 throughthe n-type emitter layer 15 and the p-type base layer 13. The trenchesGT1 to GT3 are provided periodically in a direction along the firstsurface 10T of the semiconductor part 10 (e.g., the X-direction). Forexample, the depths of the trenches GT1 to GT3 each are set in the rangeof 1 to 10 μm. For example, the spacing of mutually-adjacent trenches inthe X-direction are set in the range of 0.1 to several μm between thetrench GT1 and trench GT2, between the trench GT1 and trench GT3, andbetween the trench GT2 and trench GT3.

For example, the n-type base layer 11, the p-type base layer 13, and then-type emitter layer 15 are exposed at the side surfaces of the trenchesGT1 to GT3. The first gate electrode 40 is inside the trench GT1 andopposes the p-type base layer 13, which is positioned between the n-typebase layer 11 and the n-type emitter layer 15, with the gate insulatingfilm 41 interposed. The second gate electrode 50 is inside the trenchGT2 and opposes the p-type base layer 13, which is positioned betweenthe n-type base layer 11 and the n-type emitter layer 15, with the gateinsulating film 51 interposed. The third gate electrode 60 is inside thetrench GT3 and opposes the p-type base layer 13, which is positionedbetween the n-type base layer 11 and the n-type emitter layer 15, withthe gate insulating film 61 interposed.

For example, the trench gate structure recited above is multiplyprovided and arranged periodically along the first surface 10T of thesemiconductor part 10. For example, the spacing between the trench gatestructures is set in the range of 0.1 to several μm.

The n-type buffer layer 19 is provided between the n-type base layer 11and the collector electrode 30. The n-type buffer layer 19 includesn-type impurities with a higher concentration than the concentration ofn-type impurities in the n-type base layer 11. For example, the n-typebuffer layer 19 is provided to have an n-type impurity amount in therange of 1×10¹¹ to 1×10¹³ cm⁻² and has a thickness in the Z-direction inthe range of 0.1 to the several tens of μm. For example, the n-typebuffer layer 19 is formed by ion-implanting the n-type impurities on thesecond surface 10B side of the semiconductor part 10.

The p-type collector layer 21 is provided between the n-type bufferlayer 19 and the collector electrode 30. For example, the p-typecollector layer 21 is provided to have a p-type impurity amount in therange of 1×10¹³ to 1×10¹⁵ cm⁻² and has a thickness in the Z-direction inthe range of 0.1 to 10 μm.

For example, the p-type collector layer 21 is formed by ion-implantingp-type impurities into the semiconductor part 10 through the wholesecond surface 10B or the selected portion thereof. For example, thetotal amount of the p-type impurities is set in the range of 1×10¹³ to1×10¹⁵ cm⁻². For example, the implantation energy when forming thep-type collector layer 21 is set to be lower than the implantationenergy when forming the n-type buffer layer 19. Therefore, theimplantation depth of the p-type impurities is shallower than theimplantation depth of the n-type impurities in the n-type buffer layer19.

At the second surface 10B of the semiconductor part 10, the collectorelectrode 30 contacts the surface of the p-type collector layer 21 andis electrically connected to the p-type collector layer 21.

An operation of the semiconductor device 1 according to the embodimentwill now be described with reference to FIG. 1 , FIG. 2 , and FIGS. 3Ato 3C. FIG. 2 is a time chart showing a method of controlling thesemiconductor device 1. FIG. 2 shows the process of turning on andsubsequently turning off the semiconductor device 1. FIGS. 3A to 3C areschematic views showing the operation of the semiconductor device 1. Thecarrier density distribution inside the n-type base layer 11 is shown inFIGS. 3A to 3C. Here, the carrier density is the density including bothelectrons and holes.

FIG. 2 is a time chart showing the time-dependent change of a gatevoltage V_(MG) applied to the first gate electrode 40, a gate voltageV_(CG) applied to the second gate electrode 50, and a gate voltageV_(PG) applied to the third gate electrode 60. Here, the gate voltageV_(MG) is applied to the first gate electrode 40 via the first gate pad47 and the first gate interconnect 45. The gate voltage V_(CG) isapplied to the second gate electrode 50 via the second gate pad 57 andthe second gate interconnect 55. The gate voltage V_(PG) is applied tothe third gate electrode 60 via the third gate pad 67 and the third gateinterconnect 65.

At first, explained is the turning on process. For example, when turningon the semiconductor device 1, the gate voltages V_(MG), V_(CG), andV_(PG) that exceed the thresholds are applied to the first gateelectrode 40, the second gate electrode 50, and the third gate electrode60. Hereinbelow, applying a gate voltage that exceeds the threshold toeach gate electrode is called ON; and reducing the gate voltage of eachgate electrode to a voltage not more than the threshold is called OFF.

As shown in FIG. 2 , the first gate electrode 40, the second gateelectrode 50, and the third gate electrode 60 are turned on at a timet₁. Thereby, n-type channels are formed at the interface between thep-type base layer 13 and the gate insulating film 41, the interfacebetween the p-type base layer 13 and the gate insulating film 51, andthe interface between the p-type base layer 13 and the gate insulatingfilm 61; and the electric conduction is provided between the n-type baselayer 11 and the n-type emitter layer 15. In other words, thesemiconductor device 1 is set to the on-state.

Moreover, at portions of the first gate electrode 40, the second gateelectrode 50, and the third gate electrode 60, which are positionedinside the n-type base layer 11, n-type accumulation layers are formedat the interface between the n-type base layer 11 and the gateinsulating film 41, the interface between the n-type base layer 11 andthe gate insulating film 51, and the interface between the n-type baselayer 11 and the gate insulating film 61. The n-type accumulation layersinfluence and enhance the carrier accumulation in each region of then-type base layer 11 positioned between the gate electrodes is promoted;and the lower on-resistance can be obtained in the turned-on state.

FIG. 3A is a schematic view illustrating a carrier density distributionD₁ inside the n-type base layer 11 in such a state. By turning on thefirst gate electrode 40, the second gate electrode 50, and the thirdgate electrode 60, electrons are injected from the emitter electrode 20into the n-type base layer 11; and holes that correspond to the electroninjection are injected from the p-type collector layer 21 into then-type base layer 11 via the n-type buffer layer 19. As shown in FIG.3A, the carrier density inside the n-type base layer 11 is high on theemitter side. Thus, all of the first gate electrode 40, the second gateelectrode 50 and the third gate electrode 60 are turned on, and theturn-on time can be reduced.

Then, as shown in FIG. 2 , the third gate electrode 60 is turned off ata time t₂. Thereby, the n-type channel disappears, which is induced atthe interface between the p-type base layer 13 adjacent to the thirdgate electrode 60 and the gate insulating film 61. Therefore, theelectric conduction is disconnected between the n-type emitter layer 15and the n-type base layer 11 on the third gate electrode 60 side; andthe supply of the electrons to the n-type base layer 11 stops.Accordingly, the amount of the holes injected from the p-type collectorlayer 21 into the n-type base layer 11 via the n-type buffer layer 19also decreases.

FIG. 3B is a schematic view showing the change of the carrier densitydistribution in this process. By turning off the third gate electrode60, the carrier density on the emitter side decreases in the n-type baselayer 11; and the density distribution changes from D₁ to D₂.

Then, explained is the turn off process. As shown in FIG. 2 , the secondgate electrode 50 is turned off at a time t₃.

The n-type channel disappears, which is induced at the interface betweenthe p-type base layer 13 adjacent to the second gate electrode 50 andthe gate insulating film 51; and the electric conduction also isdisconnected between the n-type emitter layer 15 and the n-type baselayer 11. Therefore, the supply of the electrons to the second gateelectrode 50 side stops. Accordingly, the amount of the holes injectedfrom the p-type collector layer 21 into the n-type base layer 11 via then-type buffer layer 19 also decreases.

FIG. 3C is a schematic view showing the change of the carrier densitydistribution in this process. By turning off the second gate electrode50, the carrier density on the emitter side decreases further in then-type base layer 11; and the density distribution changes from D₂ toD₃.

Then, by turning off the first gate electrode 40 at a time t₄, theinjection of electrons from the n-type emitter layer 15 into the n-typebase layer 11 stops completely; and the turn-off operation starts in thesemiconductor device 1. In the example, the second gate electrode 50 isturned off before the first gate electrode 40 is turned off; therefore,the density of carriers accumulated in the n-type base layer 11 havedecreased on the emitter side. In other words, the carrier density islower than the steady state in which the first gate electrode 40 and thesecond gate electrode 50 both are turned on. The carriers that areejected in the turn-off process after turning off the first gateelectrode 40 can be reduced thereby. In other words, compared to thecase where the second gate electrode 50 is not provided, the turn-offtime can be shortened; and the turn-off loss can be reduced.

When the potential of the second gate electrode 50 is further reduced toa negative potential, a p-type inversion layer is induced at theinterface between the n-type base layer 11 and the gate insulating film51. The ejection of the holes into the emitter electrode 20 via thep-type base layer 13 can be promoted thereby. As a result, the turn-offloss can be reduced further.

In the semiconductor device 1 according to the embodiment, the turn-ontime can be shortened by appropriately controlling the third gateelectrode 60. Also, in the turn-off process, the turn-off time can beshortened by appropriately controlling the second gate electrode 50. Asa result, in the semiconductor device 1, the switching loss can bereduced while maintaining the on-resistance.

FIG. 4 is a schematic cross-sectional view showing a semiconductordevice 2 according to a modification of the embodiment.

In the semiconductor device 2 shown in FIG. 4 , four of the third gateelectrodes 60 are disposed between the first gate electrode 40 and thesecond gate electrode 50. The number of the third gate electrodes 60 isan example and is not limited to the example. The semiconductor device 2also includes a first gate pad 75 and a second gate pad 77.

As shown in FIG. 4 , the first gate electrode 40 is connected to thefirst gate interconnect 45. The second gate electrode 50 is connected tothe second gate interconnect 55. The third gate electrode 60 isconnected to the third gate interconnect 65. The second gateinterconnect 55 is linked to the first gate pad 75; and the third gateinterconnect 65 is linked to the second gate pad 77.

The semiconductor device 2 further includes a resistance element R_(D)linking the first gate interconnect 45 and the second gate interconnect55. For example, the resistance element R_(D) is a semiconductor or ametal body that has the desired resistance value. For example, theresistance element R_(D) is placed in the terminal region of thesemiconductor part 10, and provided on the first surface 10T with aninsulating film interposed. The resistance element R_(D) may be placedat the periphery of the semiconductor part 10 and may be connected tothe first gate interconnect 45 and the second gate interconnect 55 by ametal interconnect, for example.

In the semiconductor device 2, the gate voltages V_(MG) and V_(CG) arerespectively applied to the first gate electrode 40 and the second gateelectrode 50 via the first gate pad 75. The gate voltage V_(PG) isapplied to the third gate electrode 60 via the second gate pad 77.

When the semiconductor device 2 is turned on, the gate voltages V_(MG),V_(CG), and V_(PG) that are higher than the thresholds are applied tothe first gate electrode 40, the second gate electrode 50, and the thirdgate electrode 60 via the first gate pad 75 and the second gate pad 77.At this time, a delay in the rise of the voltage applied to the firstgate electrode 40 occurs due to the RC time constant due to theresistance element R_(D) and a parasitic capacitance C. Therefore, thefirst gate electrode 40 is turned on after the second gate electrode 50is turned on. Then, the third gate electrode 60 is turned off at thetime t₂. The n-type channel disappears, which is induced at theinterface between the p-type base layer 13 adjacent to the third gateelectrode 60 and the gate insulating film 61. Therefore, the electricconduction is disconnected between the n-type emitter layer 15 and then-type base layer 11 on the third gate electrode 60 side; and the supplyof the electrons to the n-type base layer 11 stops. Accordingly, theamount of the holes injected from the p-type collector layer 21 into then-type base layer 11 via the n-type buffer layer 19 also decreases. Atthis time, for example, the turn-on operation can be faster by setting anumber of the third gate electrodes larger than the number of the firstgate electrodes.

The turn-off operation will be described as follows. The first gateelectrode 40 and the second gate electrode 50 are turned off via thefirst gate pad 75. At this time, a delay in the fall of the voltageapplied to the first gate electrode 40 occurs by the RC time constantdue to the resistance element R_(D) and the parasitic capacitance C.Therefore, the first gate electrode 40 is turned off after the secondgate electrode 50 is turned off. In other words, the second gateelectrode 50 is turned off before the first gate electrode 40 is turnedoff and the turn-off process is started; and the carrier density insidethe n-type base layer 11 is reduced. As a result, the turn-off time ofthe semiconductor device 2 can be shortened; and the switching loss canbe reduced.

In the example as well, the turn-on time can be shortened byappropriately controlling the third gate electrode 60; and the turn-offtime can be shortened by appropriately controlling the off-timing of thefirst gate electrode 40 with respect to the off-timing of the secondgate electrode 50. The delay of the time for turning off the first gateelectrode 40 can be controlled by placing the resistance element R_(D)that has the desired resistance value.

FIG. 5 is a schematic cross-sectional view showing a semiconductordevice 3 according to another modification of the embodiment.

The semiconductor device 3 shown in FIG. 5 also includes the first gatepad 75 and the second gate pad 77. The semiconductor part 10 of thesemiconductor device 3 further includes an n-type barrier layer 25 (asixth semiconductor layer) positioned between the n-type base layer 11and the p-type base layer 13.

The n-type barrier layer 25 includes the n-type impurities with a higherconcentration than the concentration of n-type impurities in the n-typebase layer 11. Also, the n-type barrier layer 25 includes the n-typeimpurities with a lower concentration than the concentration of n-typeimpurities in the n-type emitter layer 15. For example, the n-typebarrier layer 25 is provided to have an n-type impurity amount in therange of 1×10¹² to 1×10¹⁴ cm⁻² and has a thickness in the Z-direction inthe range of 0.1 to several μm. For example, the n-type barrier layer 25is formed by ion-implanting the n-type impurities on the first surface10T side of the semiconductor part 10.

In the semiconductor device 3, the turn-on time can be shortened byappropriately controlling the third gate electrode 60 at turn-on.Further, when the semiconductor device 3 is turned off, by delaying theoff-timing of the first gate electrode 40 with respect to the off-timingof the second gate electrode 50, the switching loss can be reduced whilemaintaining the on-resistance. In the example, by adding the n-typebarrier layer 25, the carrier accumulation on the emitter side can bepromoted further in the n-type base layer 11 during the turn-on (i.e.,during the steady state); and the lower on-resistance is achieved. Thus,it is possible to shorten the turn-on time and the turn-off time andreduce the on-resistance more effectively by providing the n-typebarrier layer 25.

FIG. 6 is a schematic cross-sectional view showing a semiconductordevice 4 according to yet another modification of the embodiment. Thesemiconductor device 4 has the structure similar to the semiconductordevice 1 shown in FIG. 1 . The semiconductor part 10 of thesemiconductor device 4 further includes an n-type barrier layer 25between the n-type base layer 11 and the p-type base layer 13. Then-type barrier layer 25 includes n-type impurities with a concentrationhigher than a concentration of n-type impurities in the n-type baselayer 11.

Also in the semiconductor device 4, a lower on-resistance can beachieved by adding the n-type barrier layer 25.

FIG. 7 is a schematic cross-sectional view showing a semiconductordevice 5 according to other modification of the embodiment. Thesemiconductor device 5 includes the first gate electrode 40, the secondgate electrode 50, and the third gate electrode 60. The first gateelectrode 40, the second gate electrode 50, and the third gate electrode60 are electrically connected to, for example, the first gate pad 47,the second gate pad 57, and the third gate pad 67, respectively, and canbe biased independently (see FIG. 1 ).

As shown in FIG. 7 , the first gate electrode is provided in aplurality, and the second gate electrode 50 is positioned between thefirst gate electrodes 40, for example. Moreover, the first gateelectrode is disposed so as to be positioned between the second gateelectrode 50 and the third gate electrode 60, for example.

The semiconductor part 10 includes the n-type base layer 11, the p-typebase layer 13, the n-type emitter layer 15, the p-type contact layer 17,the n-type buffer layer 19, the p-type collector layer 21 and an n-typebarrier 25. The embodiment may include a structure without the n-typebarrier layer 25.

The n-type emitter layer 15 is disposed to be in contact with the gateinsulating film 41. The first gate electrode 40 is disposed so as tooppose the n-type base layer 11, the n-type barrier layer 25, the p-typebase layer 13, and the n-type emitter layer 15 with the gate insulatingfilm 41 interposed. The second gate electrode 50 is disposed so as tooppose the n-type base layer 11, the n-type barrier layer 25, the p-typebase layer 13 and the p-type contact layer 17 with the insulating film51 interposed. The third gate electrode 60 is disposed so as to opposethe n-type base layer 11, the n-type barrier layer 25, the p-type baselayer 13, and the p-type contact layer 17 with the gate insulating film61 interposed.

FIGS. 8 and 9A to 9C are schematic views showing the operation of thesemiconductor device 5. FIG. 8 is a time chart showing temporal changesof the gate voltages V_(MG), V_(CG), V_(PG), the emitter-collectorvoltage V_(CE), and the collector current I_(C). FIGS. 9A to 9C areschematic views showing carriers in the semiconductor part 10 in theperiods A, B and C shown in FIG. 8 . Then, the operation of thesemiconductor device 5 will be described with reference to FIGS. 8 and9A to 9C.

FIG. 8 shows gate voltages V_(MG), V_(CG), and V_(PG) applied to thefirst gate electrode 40, the second gate electrode 50, and the thirdgate electrode 60, respectively, during the turn-off process of thesemiconductor device 5 shifted from the on state to the off state. Thecollector current I_(C) and the emitter-collector electrode voltageV_(CE) are also shown, which are changed with the gate voltages V_(MG),V_(CG), and V_(PG). In the following description, it is assumed that thepotential of the emitter electrode 20 is at the ground potential (i.e.,zero level).

In the ON state, for example, the positive gate voltage V_(MG) equal toor higher than the threshold value is applied to the first gateelectrode 40, and the positive gate voltages V_(CG) and V_(PG) areapplied to the second gate electrode 50 and the third gate electrode 60,respectively.

As shown in FIG. 8 , in the turn-off process of the semiconductor device5, after the second gate electrode 50 and the third gate electrode 60are turned off at time t₃, the first gate electrode 40 is turned off attime t₄. For example, the gate voltage V_(CG) is decreased at time t₃from a positive voltage (for example, +15V) to a negative voltage (forexample, —15V). Further, the gate voltage V_(PG) is reduced at time t₃from a plus voltage (for example, +15V) to a zero level. Subsequently,the gate voltage VMS is reduced at time t₄, from a positive voltage (forexample, +15 V) to a zero level.

FIG. 9A shows the distribution of carriers (electrons) in thesemiconductor part 10 in the period A. In the period A, the first gateelectrode 40 is turned on, and makes it possible to inject the electronsinto the n-type barrier layer 15 and further into the n-type base layer11 from the n-type emitter layer 15 through the n-type channel inducedat the interface between the p-type base layer 13 and the gateinsulating film 41. Correspondingly, on the collector side (not shown),holes are injected from the p-type collector layer 21 into the n-typebase layer 11, and a collector current I_(C) (i.e., ON current) flows.

Further, by the gate voltages V_(CG) and V_(PG) (positive voltage)applied to the second gate electrode 50 and the third gate electrode 60,the n-type accumulation layers are induced at the interfaces between then-type base layer 11 and the gate insulating film 51, between the n-typebarrier layer 25 and the gate insulating film 51, between the n-typebase layer 11 and the gate insulating film 61, and between the n-typebarrier layer 25 and the gate insulating film 61. Thereby, the holesinjected from the p-type collector layer 21 into the n-type base layer11 further increases. As a result, the carrier amount in the n-type baselayer 11 and the n-type barrier layer 25 is increased, and makes theon-resistance lower. In the period A shown in FIG. 8 , theemitter-collector electrode voltage V_(CE) corresponding to thecollector current I_(C) is suppressed by the reduced on-resistance, andthus, the power consumption V_(CE)*I_(C) can be reduced.

In the period B shown in FIG. 8 , the second gate electrode 50 and thethird gate electrode 60 are turned off. As shown in FIG. 9B, the n-typeaccumulation layers disappear, which are induced at the interfacesbetween the n-type base layer 11 and the gate insulating film 51,between the n-type barrier layer 25 and the gate insulating film 51,between the n-type base layer 11 and the gate insulating film 61, andbetween the n-type barrier layer 25 and the gate insulating film 61.Thereby, the amount of holes injected from the p-type collector layer 21to the n-type base layer 11 is reduced. In contrast, as the first gateelectrode 40 remains in the on state, the collector current I_(C)continues to flow. That is, the electron injection from the n-typeemitter layer 15 into the n-type barrier layer 25 and the n-type baselayer 11 is maintained, and correspondingly, holes are injected from thep-type collector layer 21 into the n-type base layer 11. Further, thegate voltage V_(CG) applied to the second gate electrode 50 is anegative voltage, and induces a p-type accumulation layer at theinterfaces between the n-type base layer 11 and the gate insulating film51, between the n-type barrier layer 25 and the gate insulating film 51,and between the p-type base layer 13 and the gate insulating film 51.Thereby, the hole ejection path is formed from the n-type base layer 11and the n-type barrier layer 25 to the p-type contact layer 17. Thus,the holes are ejected from the n-type base layer 11 and the n-typebarrier layer 25 to the emitter electrode 20, and the carrier amount inthe n-type base layer 11 and the n-type barrier layer 25 is furtherreduced.

As a result, the collector current I_(C) flows under the increasedon-resistance by the reduced carrier amount in the n-type base layer 11and the n-type barrier layer 25. Thus, the emitter-collector voltageV_(CE) increases, and the power consumption V_(CE)*I_(C) increasesslightly.

In the period C shown in FIG. 8 , the first gate electrode 40 is furtherturned off. For example, when the gate voltage VMS is decreased in orderto turn off the first gate electrode 40, the potential of the first gateelectrode 20 is not immediately lowered to the off potential. Forexample, while discharging the parasitic capacitor of the first gateelectrode 40, the collector current I_(C) is maintained to besubstantially constant.

Subsequently, when the potential of the first gate electrode 40 startsto be reduced, the n-type channel gradually disappears at the interfacebetween the p-type base layer 13 and the gate insulating film 41. Then,the collector current I_(C) starts to decrease; and theemitter-collector voltage V_(CE) starts to rise. The emitter-collectorvoltage V_(CE), for example, once overshoots, and then, becomes theconstant off-voltage.

During this period, the carriers in the n-type base layer 11 and then-type barrier layer 25 are ejected to the emitter electrode 20 and thecollector electrode 30, and the n-type base layer 11 and the n-typebarrier layer 25 are depleted.

As shown in FIG. 9C, the holes in the n-type base layer 11 and then-type barrier layer 25 are ejected to the emitter electrode 20. At thistime, as the negative voltage is applied to the second gate electrode 50(for example, V_(CG)=−15V), the p-type accumulation layer is induced atthe interfaces between the n-type base layer 11 and the gate insulatingfilm 51, and between the n-type barrier layer 25 and the gate insulatingfilm 51. Thus, the holes in the n-type base layer 11 and the n-typebarrier layer 25 are quickly ejected to the emitter electrode 20 throughthe p-type accumulation layer and the p-type base layer 13.

Further, a negative voltage is applied to the first gate electrode 40(for example, V_(MG)=−15V) to induce a p-type accumulation layer at theinterfaces between the n-type base layer 11 and the gate insulating film41, and between the n-type barrier layer 25 and the gate insulating film41. Thereby, the hole ejection is enhanced from the n-type base layer 11and the n-type barrier layer 25 to the emitter electrode 20 through thep-type accumulation layer.

Further, in the period D shown in FIG. 8 , the collector current I_(C)decreases to the zero level, and the semiconductor device 5 is turnedoff at time t₅.

In the embodiment, the carrier ejection from the n-type base layer 11and the n-type barrier layer 25 can be enhanced by providing the secondgate electrode 50 and the third gate electrode 60, and the turn-offperiod C can be shortened.

Moreover, before applying the off voltage (for example, V_(MG)=−15V) tothe first gate electrode 40 at time t₄, the second gate electrode 50 andthe third gate electrode 60 are turned off at time t₃. Thereby, it ispossible in the period B to start the turn-off period C after thecarriers are reduced beforehand in the n-type base layer 11 and then-type barrier layer 25. As a result, the turn-off period C can befurther shortened.

For example, the variation of the emitter-collector voltage V_(CE)indicated by a broken line in FIG. 8 exhibits the characteristicswithout the second gate electrode 50 and the third gate electrode 60. Itis found that the turn-off period can be shortened by providing thesecond gate electrode 50 and the third gate electrode 60 as comparedwith the temporal change indicated by the solid line that shows thecharacteristics of the emitter-collector voltage V_(CE) according to theembodiment.

The power consumption V_(CE)*I_(C) in the turn-off period C depends onthe length thereof, and the switching loss can be reduced by shorteningthe turn-off period C. In the semiconductor device 5 according to theembodiment, the on-resistance and the switching loss can be reduced byappropriately controlling the second gate electrode 50 and the thirdgate electrode 60.

For example, in the semiconductor device 5, the power consumptionV_(CE)*I_(C) is slightly increased in the period B by reducing thecarrier amount in the n-type base layer 11 and the n-type barrier layer25. However, the power consumption V_(CE)*I_(C) is reduced moreeffectively by the shortening of the turn-off period C, and theswitching loss can be reduced as a whole.

Moreover, by applying a negative voltage to the third gate electrode 60(for example, V_(PG)=−15V), the hole ejection from the n-type base layer11 and the n-type barrier layer 25 can be further enhanced. However,such control causes a significant decrease of the carrier amount in then-type base layer 11 and the n-type barrier layer 25 in the period B,for example, and increases the power consumption V_(CE)*I_(C), resultingin the increase of the switching loss. Thus, by appropriately arrangingand controlling the second gate electrode 50 and the third gateelectrode 60 according to this embodiment, it is possible to reduce theon-resistance and the switching loss of the semiconductor device.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the invention.

What is claimed is:
 1. A semiconductor device, comprising: asemiconductor part provided with first and second surfaces, the secondsurface being positioned on a side opposite to the first surface; afirst electrode provided on the first surface; a second electrodeprovided on the second surface; first to third control electrodes eachprovided at the second surface side of the semiconductor part with atrench gate structure; and first to third control pads electricallyconnected to the first to third control electrodes, respectively, thefirst control pad being electrically connected to at least a firstcontrol electrode, the second control pad being electrically connectedto at least a second control electrode, the third control pad beingelectrically connected to at least a third control electrode, the firstto third control pads being electrically isolated from each other sothat, while a prescribed voltage is applied between the first and secondelectrodes, a first on-voltage and a first off-voltage are appliedbetween the first control pad and the second electrode, the firston-voltage being applied until the first off-voltage being applied, thefirst on-voltage being higher than a threshold voltage of the firstcontrol electrode, the first off-voltage being lower than the thresholdvoltage of the first control electrode, a second on-voltage and a secondoff-voltage are applied between the second control pad and the secondelectrode, the second on-voltage being applied until the secondoff-voltage being applied, the second off-voltage being applied beforethe first off-voltage being applied between the first control pad andthe second electrode, the second on-voltage being higher than athreshold voltage of the second control electrode, the secondoff-voltage being lower than the threshold voltage of the second controlelectrode, and a third on-voltage and a third off-voltage are appliedbetween the third control pad and the second electrode, the thirdon-voltage being applied until the third off-voltage being applied, thethird off-voltage being applied before the first off-voltage beingapplied between the first control pad and the second electrode, thethird on-voltage being higher than a threshold voltage of the thirdcontrol electrode, the third off-voltage being lower than the thresholdvoltage of the third control electrode and higher than the secondoff-voltage.
 2. The device according to claim 1, wherein the firstcontrol electrode, the second control electrode, and the third controlelectrode are arranged along the second surface of the semiconductorpart, and the third control electrode being provided between the firstcontrol electrode and the second control electrode.
 3. The deviceaccording to claim 2, wherein, the semiconductor part includes a firstportion and a second portion, the first portion of the semiconductorpart being provided between the first control electrode and the thirdcontrol electrode, the second portion of the semiconductor part beingprovided between the second control electrode and the third controlelectrode, and the first and second portions of the semiconductor parteach having a width in a first direction directed from the first controlelectrode toward the second control electrode, the width of the firstand second portions being greater than 0.1 micrometers.
 4. The deviceaccording to claim 1, wherein the second electrode is electricallyconnected to a fixed potential; the first and second off-voltages are−1.5 V with respect to the fixed potential; and the third off-voltageare 0 V with respect to the fixed potential.
 5. The device according toclaim 2, wherein the first to third control electrodes are providedbetween the first electrode and the second electrode, and thesemiconductor part includes an n-type base layer, a p-type base layer,and an n-type emitter layer, the n-type base layer being providedbetween the first electrode and the second electrode, the n-type baselayer being provided with a thickness in a second direction directedfrom the first electrode to the second electrode so that a prescribedbreakdown voltage is provided between the first and second electrodes,the thickness of the n-type base layer being in a range of 1 to 1000micrometers, the p-type base layer being provided between the n-typebase layer and the second electrode, the p-type base layer beingprovided respectively between the first control electrode and the thirdcontrol electrode and between the second control electrode and the thirdcontrol electrode, the p-type base layer being provided with a thicknessin the second direction of 0.1 to several micrometers and including ap-type impurity with a concentration in a range of 1×10¹² to 1×10¹⁴cm⁻², the n-type emitter layer being provided between the p-type baselayer and the second electrode, the n-type emitter layer being providedwith a thickness in the second direction of 0.1 to several micrometersand including a n-type impurity with a concentration in a range of1×10¹⁴ to 1×10¹⁶ cm⁻².
 6. The device according to claim 5, wherein thesemiconductor part further includes a p-type collector layer and an-type buffer layer, the p-type collector layer being provided betweenthe first electrode and the n-type base layer, the n-type buffer layerbeing provided between the n-type base layer and the p-type collectorlayer; the p-type collector layer includes a p-type impurity distributedin the second direction with a first peak concentration positioned at afirst depth from the first surface of the semiconductor part; and then-type buffer layer includes an n-type impurity distributed in thesecond direction with a second peak concentration positioned at a seconddepth from the first surface of the semiconductor part, the first depthbeing shallower than the second depth.
 7. The device according to claim1, wherein multiple first control electrodes, multiple second controlelectrodes and multiple third electrodes are provided at the secondsurface side of the semiconductor part, and a number of the multiplefirst electrodes is greater than a number of the multiple second controlelectrodes and greater than a number of multiple third controlelectrodes.
 8. A semiconductor device, comprising: a semiconductor partprovided with first and second surfaces, the second surface beingpositioned on a side opposite to the first surface; a first electrodeprovided on the first surface; a second electrode provided on the secondsurface; first to third control electrodes each provided at the secondsurface side of the semiconductor part with a trench gate structure; anelectrical resistance electrically connecting the first controlelectrode and the second control electrode; and a first control padelectrically connected via the electrical resistance to the firstelectrode, the first control pad being electrically connected to thesecond electrode; and the second control pad electrically connected tothe third control electrode.
 9. The device according to claim 8, whereinthe first control pad is electrically isolated from the second controlpad so that, while a prescribed voltage is applied between the first andsecond electrodes, a first on-voltage and a first off-voltage areapplied between the first control pad and the second electrode, thefirst on-voltage being applied until the first off-voltage beingapplied, the first on-voltage being higher than threshold voltages ofthe first and second control electrodes, the first off-voltage beinglower than the threshold voltages of the first and second controlelectrodes, the first on-voltage and the first off-voltage being appliedto the first control electrode with a delay due to the electricalresistance, and a second on-voltage and a second off-voltage are appliedbetween the second control pad and the second electrode, the secondon-voltage being applied until the second off-voltage being applied, thesecond off-voltage being applied before the first off-voltage beingapplied between the first control pad and the second electrode, thesecond on-voltage being higher than a threshold voltage of the thirdcontrol electrode, the second off-voltage being lower than the thresholdvoltage of the third control electrode and higher than the firstoff-voltage.